As is known, the porosity of silica bricks may range between about 15% and 25%, the number of closed pores in the bricks being less than 1% because of their low glass content. As a result, silica bricks do not have an impervious fired skin, their permeability being higher than chamotte bricks having the same total porosity. They also tend to absorb liquids and gases and, compared with chamotte bricks, have a relatively rough surface which promotes condensation and adhesion of tar and graphite in coke ovens. Such silica bricks are particularly susceptible to the penetration of hot liquid pitch or tar. When silica bricks are used in coke ovens, their structure disintegrates in the pores of the bricks and relatively large areas of brickwork wear out prematurely in certain zones of the coke oven.
It is known to seal silica bricks, particularly silica bricks used in coke ovens, by spraying or brushing on compounds which consist mainly of a refractory material such as silicon dioxide, mullite, corundum or the like, fluxing agents such as glass powder, water-glass, borax or phosphate and plasticizers which consist of clay, glue, dextrin or the like. Because of their relatively high refractory properties, these compounds do not adhere well to hot walls, even when sprayed on. They are used primarily for repairing cracked, leaky brickwork areas to provide only temporary improvement.
In another known method, very high temperature flames are used (e.g., an acetylene/oxygen flame) to heat the surfaces of the silica walls to a temperature such that refractory material applied to the walls by blasting adheres thereto by fusion. This method is extremely expensive and has not become generally accepted.
The prior art also includes glazes and protective coatings for refractory products which involve the use of refractory material or extremely expensive raw materials. Such protective coatings are disclosed, for example, in German Offenlegungsschrift No. 29 37 997, in which silicon nitride or oxynitride is used for ceramic hot conductors. German Offenlegungsschrift No. 16 46 497 relates to a process for coating articles with silicon carbide to produce a hard wear-resistant surface, wherein a metal or carbon article at a temperature of 1150.degree. C. is dipped into organic silicon compounds. German Offenlegungsschrift No. 17 71 733 discloses a refractory protective coating which is said to have a thickness of 20-50 millimeters and contains 40%-60% aluminum oxide. The coating compound disclosed in the German Offenlegungsschrift No. 17 71 623, is also refractory, containing only up to 4% of a mineral mixture which is not refractory. German Auslegeschrift No. 22 23 142 relates to an alkali-free silicon dioxide suspension which does not involve a glaze. The coating compound disclosed in German Auslegeschrift No. 13 02 596 also has a very high solids content. The melting point of the coating, which is intended for use in coating porous chamotte bricks, is said to be 150.degree. higher than the melting temperature of the brick itself.
All of the foregoing prior art methods are either extremely expensive or are not altogether satisfactory for their intended purpose.